Method of heat treating high speed steels



March 2, 1965 P. A. GLENN METHOD OF HEAT TREATING HIGH swan srms FiledSept. 21, 1962 \alezozzm 1005 OF DEGREE-5 F- lo :2, l4

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United States Patent Perry A. Glenn, Rockford, IlL, assignor to lpsenindustries, Inc., Cherry Valley, Ill., a corporation of Illinois FiledSept. 21, 1962, Ser. No. 225,333 4 Claims. (Cl. 148-46) This inventionrelates to the heat treating of high speed steels and, moreparticularly, to a heat treating method in which the work first ispreheated at one temperature and then further heated at a highertemperature.

The general object of the invention is to provide a novel heat treatingmethod which is rapid, inexpensive and capable of being performedautomatically, which minimizes distortion of the work and which retainsa bright surface on the work without scaling.

A more detailed object is to achieve the foregoing by performing theheat treating at subatmospheric pressures with the preheat taking placeat a high degree of vacuum and the high heat taking place at an absolutepressure somewhat higher but substantially below atmospheric and in anon-oxidizing atmosphere.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in which FIGURE 1 is a schematic cross sectionalview of a heat treating furnace suitable for carrying out the novelmethod of the present invention.

FIG. 2 is a chart of curves showing typical pressures and temperaturesused in practicing the invention.

The present invention contemplates the provision of a novel method bywhich high speed steel such as tool steel may be heat treated whilemaintaining a bright surface on the workpiece. In general, this methodutilizes both vacuum heating and heating in a controlled atmosphere withthe work being preheated in a high degree of vacuum and the final heattaking place in the presence of a nonoxidizing gas at a pressure belowatmospheric. Moreover, the work is cooled in a non-oxidizing atmospherewith the result that the surface of the work is substantially as brightafter heat treating as it was before.

More specifically, the work is placed in a suitable chamber which may beselectively heated and which is arranged either to be evacuated or tocontain a controlled atmosphere. With the Work in the chamber, thelatter is evacuated and, after the absolute pressure within the chamberis at least as low as 1090 microns of mercury, the first heating step isbegun. The evacuation continues until the pressure in the chamber isbelow 100 microns of mercury and preferably below 20 microns and,simultaneously, the temperature within the chamber is raised to theusual preheat temperature for the particular work. This temperature isthe so-called critical temperature at which the steel becomes largelyaustenitic and the carbides begin to go into solution. The pressure andtemperature are maintained at these values until the work is heatedthrough.

Next, a non-oxidizing gas such as hydrogen is admitted to the chamber toraise the pressure to between 100 and 200 microns of mercury, the lowerpart of this range being preferred, and the temperature is raised to apoint about 1000 degrees above the critical temperature. Theseconditions are maintained until the work is heated through at the newtemperature and then the supply of heat is shut off and additionalquantities of non-oxidizing gas are admitted to the chamber therebyraising the pressure to atmospheric. The supply of gas is continueduntil the work is cooled to about 208 degrees F. when it is removed fromthe chamber.

An apparatus suitable for performing the novel method 3,171,759 PatentedMar. 2, 165

is shown in FIG. 1 and includes a hollow, cylindrical vessel 10 which issupported on its side by a base 11 and is cooled by a peripheral waterjacket 12. Within the vessel is a refractory bafiie 13 which defines theheat treating chamber 14 where the work 15 is supported on a rollerplatform 16. The upper and lower walls of the bafiie are formed withopenings 1'7 and 18 respectively so that an atmosphere within the vessel10 may be circulated across the work. To circulate the atmosphere, acentrifugal fan 19 is keyed to the shaft of a motor 29 mounted on top ofthe vessel with the shaft projecting vertically through the vessel wall.\Vith this arrangement, the atmosphere is discharged radially by the fanand follows the path indicated by the arrows, that is, down along theoutside of the baflle 13, into the chamber 14 through the opening 18 andaxially back to the fan through the opening 17.

In the chamber 14, the work is heated by suitable radiant heatingelements 21 which may be of the electrical type and which extendhorizontally across the chamber adjacent the upper and lower walls ofthe baffle 13. Radiation of heat through the openings 17 and 18 to theoutside of the battle is prevented by refractory deflectors 22 and 23which are spaced from but cover the openings. Preferably, the outersurface of the side walls of the baffle 13 are cooled by water flowingthrough a tube 24 coiled around the battle.

To evacuate the interior of the vessel 10 and hence the chamber 14, amechanical pump 25 and a suitable oil diffusion pump 26 are connected inseries and communicate with the interior of the vessel. Thus, thediffusion pump is at one end of a conduit 2'7 whose other end projectsthrough the wall of the vessel and the outlet of the diffusion pump isconnected by a second conduit 28 to the inlet of the mechanical pump.Rough evacuation of the vessel 10 is effected by the mechanical pump 25alone while the final evacuation is achieved by the diffusion pumpacting with the mechanical pump. A suitable valve (not shown) may bedisposed in the conduit 27 and controlled by a power actuator 29 to holdthe vacuum in the vessel. The non-oxidizing gas, when required, issupplied from a suitable source 36 through a pipe 31 which projects intothe vessel 10 and is controlled by a valve 32.

A typical example of the novel method is illustrated in FIG. 2 in whichthe solid line represents temperature in degrees Fahrenheit and thebroken line represents absolute pressure in microns of mercury, bothbeing plotted against time in minutes and a logarithmic scale being usedfor pressure. In this example, the work was an M-3 high speed steel withabout a one-inch maximum cross section and, after it was placed in thechamber 14, the mechanical pump 25 was started with the vacuum valveopen to initiate the the evacuation of the vessel 10. When the absolutepressure in the vessel was lowered to about 1000 microns, the heatingelements 21 were energized to start raising the temperature of the work.At about the same time, the diffusion pump 26 started in operation sothat, after about 20 minutes from the start of the operation, theabsolute pressure in the vessel 10 was only a few microns. At that time,the temperature had been raised to 1550 degrees Fahrenheit and thispressure and temperature were maintained for approximately ten minutesafter which the work was heated through.

Next, the valve 32 was opened to admit nitrogen to the vessel 10 andraise the pressure almost instantaneously to microns, the fan 1? havingbeen started to circulate the nitrogen through the chamber 14 and acrossthe work. At the same time, the heating elements 21 were furtherenergized to raise the temperature of the work to 2225 degreesFahrenheit and held until the work was heated through, this requiring atotal of about fifteen minutes during which time the pressure wasmaintained at 100 microns. Finally, the valve 32 was opened enough toraise the pressure in the vessel to atmospheric and the heating elementswere deenergized to permit the work to cool in the nitrogen atmosphere.In about fifteen minutes, the temperature of the work was reduced toabout 300 degrees Fahrenheit after which it was removed from the vessel.

In practicing the invention, the preheating may be started any time thepressure in the vessel is 1000 microns or less but, where thetemperature control is responsive to an atmospheric indicator whichoperates on a logarithmic scale, better control is achieved if thepreheat is begun when the pressure is between and microns. For thepreheat, the pressure should be lowered to at least 100 microns and heldthere during the balance of the preheat. The preferred practice,however, is to reduce the pressure further to a value below 10 microns.

As stated above, the preheat temperature is that at which the steel islargely austenitic and the carbides be gin to go into solution. Thistemperature varies for different steels and is determined from theconventional S curve for the particular composition of the work. Thehigh heat temperature is the one normally used for heat treating eachtype of steel and usually is between 700 and 1000 degrees above thepreheat temperature, the precise temperature depending upon theproperties desired in the end product as is well known in the art. Thepressure for the high heat is substantially below atmospheric andpreferably does not exceed 200 microns, 100 microns being suitable formost applications.

In some instances, outgassing will occur, this being the removal ofsurface contaminants and the dissociation of unwanted oxides in thework, and it usually takes place after the pressure in the vessel 10 hasbeen lowered below 1000 microns. Such outgassing momentarily raises thepressure in the vessel and, for this reason, it is desirable to have theheating elements controlled by the pressure indicator so that theheating of the work may be interrupted during this temporary increase inpressure.

With the foregoing method, high speed steels may be heat treated in onecontinuous operation and in a single zone. This provides accuratecontrol over the process and minimizes distortion of the work. Moreover,there is no scaling of the work which retains a bright surfacethroughout the heat treating operation. The entire process may beperformed rapidly, inexpensively and automatically.

I claim as my invention:

1. The method of heat treating high speed steel, said method comprisingthe steps of placing a workpiece in a chamber, evacuating said chamberto an absolute pressure on the order of 20 microns of mercury,heating'said chamber after the pressure therein is below 1000 microns ofmercury to approximately the critical temperature of the steel, afterthe temperature of the workpiece has been raised to said criticaltemperature introducing nitrogen to said chamber to raise the pressuretherein to approxi mately 100 microns of mercury and simultaneouslyraising the temperature in the chamber to a point substantially abovesaid critical temperature, and, after the temperature of the workpiecehas been raised, further introducing nitrogen to said chamber to raisethe pressure to atmospheric and cool the workpiece.

2. The method of heat treating high speed steel, said method comprisingthe steps of placing a workpiece in a chamber, evacuating said chamberto an absolute pressure less han mi ons Q mer ry eat n sa Chamber afterthe pressure therein is below 1000 microns of mercury to a preheattemperature of approximately 1550 degrees B, after the temperature ofthe workpiece has been raised to said preheat temperature introducing anonoxidizing gas to said chamber to raise the pressure therein toapproximately microns of mercury and simultaneously raising thetemperature in the chamber to a temperature of approximately 2200degrees F., and, after the temperature of the workpiece has been raisedto the temperature of the chamber, further introducing a non-oxidizinggas to said chamber to raise the pressure to atmospheric and cool theworkpiece.

3. The method of heat treating high speed steel, said method comprisingthe steps of placing a workpiece in a chamber, evacuating said chamberto an absolute pressure on the order of 20 microns of mercury, heatingsaid chamber after the pressure therein is below 1000 microns of mercuryto approximately the critical temperature of the steel, after thetemperature of the workpiece has been raised to said criticaltemperature introducing a nonoxidizing gas to said chamber to raise thepressure therein to approximately 100 microns of mercury andsimultaneously raising the temperature in the chamber to a pointsubstantially above said critical temperature, and, after thetemperature of the workpiece has been raised, further introducing anon-oxidizing gas to said chamber to raise the pressure to atmosphericand cool the workpiece.

4. The method of heat treating high speed steel, said method comprisingthe steps of placing a workpiece in a chamber, evacuating said chamberto an absolute pressure of not more than 100 microns of mercury, heatingsaid chamber after the pressure therein is below 1000 microns of mercuryto approximately the critical temperature of the steel, after thetemperature of the workpiece has been raised to said criticaltemperature introducing a non-oxidizing gas to said chamber whilemaintaining the pressure therein substantially below atmospheric andsimultaneously raising the temperature in the chamber to a pointsubstantially above said critical temperature, and, after thetemperature of the workpiece has been raised, further introducing anon-oxidizing gas to said chamber to raise the pressure to atmosphericand cool the workpiece.

References Cited in the file of this patent UNITED STATES PATENTS2,565,360 Du l g- 2 1.951

1. THE METHOD OF HEAT TREATING HIGH SPEED STEEL, SAID METHOD COMPRISINGTHE STEPS OF PLACING A WORKPIECE IN A CHAMBER, EVACUATING SAID CHAMBERTO AN ABSOLUTE PRESSURE ON THE ORDER OF 20 MICRONS OF MERCURY, HEATINGSAID CHAMBER AFTER THE PRESSURE THEREIN IS BELOW 1000 MICRONS OF MERCURYTO APPROXIMATELY THE CRITICAL TEMPERATURE OF THE STEEL, AFTER THETEMPERATURE OF THE WORKPIECE HAS BEEN RAISED TO SAID CRITICALTEMPERATURE INTRODUCING NITROGEN TO SAID CHAMBER TO RAISE THE PRESSURETHEREIN TO APPROXIMATELY 100 MICRONS OF MERCURY AND SIMULTANEOUSLYRAISING THE TEMPERATURE IN THE CHAMBER TO A POUNT SUBSTANTIALLY ABOVESAID CRITICAL TEMPERATURE, AND, AFTER THE TEMPERATURE OF THE WORKPIECEHAS BEEN RAISED, FURTHER INTRODUCING NITROGEN TO SAID CHAMBER TO RAISETHE PRESSURE TO ATMOSPHERIC AND COOL THE WORKPIECE.